serial: qcom-geni: fix hard lockup on buffer flush

The Qualcomm GENI serial driver does not handle buffer flushing and used
to continue printing discarded characters when the circular buffer was
cleared. Since commit 1788cf6a ("tty: serial: switch from circ_buf
to kfifo") this instead results in a hard lockup due to
qcom_geni_serial_send_chunk_fifo() spinning indefinitely in the
interrupt handler.

This is easily triggered by interrupting a command such as dmesg in a
serial console but can also happen when stopping a serial getty on
reboot.

Implement the flush_buffer() callback and use it to cancel any active TX
command when the write buffer has been emptied.

Reported-by: Douglas Anderson <dianders@chromium.org>
Link: https://lore.kernel.org/lkml/20240610222515.3023730-1-dianders@chromium.org/


Fixes: 1788cf6a ("tty: serial: switch from circ_buf to kfifo")
Fixes: a1fee899 ("tty: serial: qcom_geni_serial: Fix softlock")
Cc: stable@vger.kernel.org	# 5.0
Signed-off-by: Johan Hovold <johan+linaro@kernel.org>
Link: https://lore.kernel.org/r/20240704101805.30612-3-johan+linaro@kernel.org


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>